1. Field of the Invention
The present invention relates to an optical disk drive for reading and/or writing data from/on an optical disk as a storage medium using a light beam, and more particularly relates to the focus control of the light beam.
2. Description of the Related Art
In optical disk technologies, data can be read out from a rotating optical disk by irradiating the disk with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disk.
On a read-only optical disk, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disk. On the other hand, on a rewritable optical disk, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on such a rewritable optical disk, data is written there by irradiating the optical disk with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits, the depth of the tracks, and the thickness of the recording material film are all smaller than the thickness of the optical disk substrate. For that reason, those portions of the optical disk, where data is stored, define a two-dimensional plane, which is sometimes called an “information storage plane”. However, considering that such an “information storage plane” has a physical dimension in the depth direction, too, the term “information storage plane” will be replaced herein by another term “information storage layer”. Every optical disk has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To write data on a recordable optical disk or to read data that is stored on such a disk, the light beam always needs to maintain a predetermined converging state on a target track on an information storage layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information storage layer such that the focus position (or converging point) of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disk such that the light beam spot is always located right on a target track.
Various types of optical disks such as DVD-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R have become more and more popular these days as storage media on which a huge amount of information can be stored at a high density. Optical disk drives compatible with those optical disks use an optical lens (i.e., an objective lens) with a numerical aperture (NA) of 0.6. Recently, however, in order to further increase the maximum densities and capacities of the optical disks, next-generation optical disks, including Blu-ray Disc (BD), have been under research and development and have already been put on the market. They suggest that an optical lens with an NA of at least 0.8 be used for such next-generation optical disks.
FIG. 1 schematically illustrates how an optical disk 1 is irradiated with a light beam 200 that has been converged by an objective lens 23. The optical disk 1 is rotating at a high velocity during a read/write operation. Therefore, to perform a high-precision focus control on the rotating optical disk 1, the degree of convergence of the light beam 200 on the information storage layer needs to be detected, while at the same time, the focus position needs to be adjusted such that the focal point of the light beam 200 is always on the information storage layer. Such focus position adjustment may be done by moving the objective lens 23 back and forth along its optical axis.
Actually, the surface of an optical disk 1 is not perfectly flat but is normally slightly warped. That is why the portion of the optical disk 1 being irradiated with the light beam 200 vibrates up and down at a high velocity, albeit slightly (e.g., on the order of several hundreds of micrometers), as the optical disk 1 turns. For that reason, if the objective lens 23 for converging the light beam 200 were fixed at the same position, then the light beam 200 would be sometimes out of focus with the information storage layer of the optical disk 1. Such a vertical vibration (i.e., the out-of-plane vibration) of the irradiated portion of the rotating optical disk 1 will be referred to herein as the “disk flutter” of the optical disk and its magnitude as the “amplitude of disk flutter”. The maximum allowable amplitude of the disk flutter of an optical disk is defined by optical disk standard specifications.
To always keep the focal point of the light beam 200 located right on the information storage layer of the optical disk 1 even with such disk flutter, the position of the objective lens 23 (i.e., its position in the axial direction) needs to be controlled in quick response to a focus error signal representing the magnitude of positional shift of the focal point of the light beam 200 from the information storage layer of the optical disk 1. Hereinafter, the basic operation of focus control will be described.
FIG. 2 is a graph showing the curve of a focus error signal. In FIG. 2, the ordinate represents the amplitude of the focus error signal and the abscissa represents the focus position of the light beam. If there is a good distance from the focus position of the light beam to the information storage layer of the optical disk, then the focus error signal has zero amplitude. However, as the focus position of the light beam approaches the information storage layer of the optical disk, the amplitude of the focus error signal has non-zero values in a certain range, thereby making an S-curve there. In FIG. 2, the range of the S-curve (i.e., the upper and lower limits thereof) is pointed by the arrows.
When a focus servo control is activated and its control loop is closed, the position of the objective lens 23 gets finely adjusted so as to make the focus error signal as close to zero as possible. However, even when there is a long distance from the focus position of the light beam to the information storage layer of the optical disk 1, the focus error signal also has zero amplitude. That is why before the focus servo control is activated, the focus position of the light beam needs to be brought close to the information storage layer of the optical disk to the point that the focus error signal shows the S-curve. That is to say, to activate the focus servo control, the focal point of the light beam needs to be brought sufficiently close to the information storage layer of the optical disk and the S-curve of the focus error signal needs to be detected by moving the objective lens 23 along its optical axis first. After the S-curve of the focus error signal has been detected in this manner, the focus servo control is activated at a good timing, thereby getting a focus control started. Such an operation of looking for the position where the focus error signal shows the S-curve by changing the positions of the objective lens 23 along its optical axis and then activating the focus servo control on detecting the S-curve will be referred to herein as a “focus finding operation”.
To get the focus finding operation done in a short time, the objective lens 23 should be moved at a high velocity along the optical axis. However, the objective lens 23 also has mass. That is why if the objective lens 23 has been moved quickly until the focus error signal shows the S-curve, the objective lens 23 cannot be stopped the instant the S-curve is detected. If the moving velocity of the objective lens 23 is high, then the focal point of the light beam may pass the information storage layer of the optical disk 1 and reach a range where the focus error signal shows no S-curve (i.e., the range where the focus error signal is zero). In that case, even if the focus servo control is carried out, the focal point of the light beam cannot keep up with the information storage layer. Since this is a state in which the focus servo has failed, the focus finding operation needs to be retried.
To avoid such a failure of the focus finding operation, the moving velocity of the objective lens 23 could be decreased. In that case, however, it would take too much time to get the focus finding operation done. In view of these considerations, to shorten the time it takes to get the focus finding operation done, it was proposed that the moving velocities of the objective lens be changed in two stages (see Patent Document No. 1).
Hereinafter, a conventional technique of getting the focus finding operation done more quickly will be described with reference to FIG. 3, which is a waveform chart showing how the focus finding operation is carried out in a conventional optical disk drive.
Portion (a) of FIG. 3 shows the focus positions of a light beam during the focus finding operation. In portion (a) of FIG. 3, the abscissa represents the time, thereby showing where the focal point of the light beam passes with time until the focal point reaches the information storage layer of the optical disk. Portion (b) of FIG. 3 shows a focus error signal.
In the example shown in FIG. 3, the focal point of the light beam is relatively far away (e.g., at a retracted position) from the information storage layer of the optical disk up to a time t1, when the focal point starts to move quickly toward the information storage layer. This is because a focus finding instruction is output at the time t1 as shown in portion (d) of FIG. 3 and the actuator starts moving the objective lens toward the optical disk 1 at a high velocity in response to this instruction.
As the objective lens is getting closer to the optical disk, the focal point of the light beam is also getting closer and closer to the information storage layer. And when the focal point of the light beam is sufficiently close to the information storage layer, a portion of the light beam is reflected by the optical disk. By detecting this reflected light, a read signal (such as an RF signal) and a focus error signal can be generated. When the intensity of the reflected light (as represented by the RF signal) becomes at least equivalent to the reference voltage at a time t2, the optical disk detection signal rises to High level as shown in portion (c) of FIG. 3. In this optical disk drive, when the level of the optical disk detection signal rises, the moving velocity of the objective lens decreases as shown in portion (a) of FIG. 3.
When the focal point of the light beam reaches the information storage layer of the optical disk as shown in portion (a) of FIG. 3, a zero cross point is detected in the S-curve of the focus error signal as shown in portion (b) of FIG. 3. At the time when this zero cross point is detected, the focus servo control loop is closed and the focus finding operation is completed. Once the focus finding operation has been done, the position of the objective lens is always controlled such that the focus error signal becomes as close to zero as possible. That is why even if the information storage layer of the optical disk waves vertically to a certain degree, the focal point of the light beam can still keep up with the information storage layer.
According to the conventional technique described above, the objective lens is moved toward the optical disk quickly until the optical disk is detected by the reflected light and then moved more slowly once the presence of the optical disk has been sensed, thereby trying to get the focus finding operation done more accurately but more quickly.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2-76128 (see FIG. 2)